UM-enabled mailbox migration to Exchange Online fails

Symptoms

When you try to migrate a Unified Messaging (UM)-enabled mailbox from on-premises Microsoft Exchange Server to Microsoft Exchange Online, the migration fails with an error that resembles the following:

Mailbox 'USER' in the source forest is currently enabled for Unified Messaging but it can't be enabled for Unified Messaging in the target forest for the following reason: Unified Messaging isn't available in the target forest. Please fix the problem or disable the mailbox for Unified Messaging before you try the operation again. + CategoryInfo : InvalidArgument: (USER:MailboxOrMailUserIdParameter) [New-MoveRequest], RecipientTask Exception + FullyQualifiedErrorId : [Server=Server,RequestId=RequestId,TimeStamp=TimeStamp] [FailureCategory=Cmdlet-RecipientTaskException] xxxxxxxx,Microsoft.Exchange.Management.RecipientTasks. NewMoveRequest

Cause

This issue occurs because Exchange Online cannot find a valid target forest for which you are trying to migrate.

Resolution

To resolve this issue, follow these steps:

Make sure that the user has an Exchange Online license.

Confirm that the value of the LicenseReconciliationNeeded property is False. To do this, run the following Microsoft Online Services Module for Windows PowerShell cmdlets in order:

Connect-MsolService

Get-MsolUser -UserPrincipalName UPN |fl LicenseReconciliationNeeded

Note If the value is True, assign an Exchange Online license to the user account. Doing this operation resets the LicenseReconciliationNeeded property to False and enables the migration to continue.

Map the UM mailbox policy on the cloud and on the on-premises Exchange server. To do this, follow these steps: